1. Field of the Invention
2. Description of the Related Art
The present invention generally relates to the use of sensors to monitor the concentration of a chemical species in bodily fluids. More specifically, the present invention relates to the use of sensors to monitor glucose levels, and/or other parameters in a fluid, including flow rate within a lumen of an endoluminal implant such as a stent or other type of endovascular conduit.
Diabetes mellitus is a serious medical condition affecting approximately 10.5 million Americans, in which the patient is not able to maintain blood glucose levels within the normal range (normoglycemia). Approximately 10% of these patients have insulin-dependent diabetes mellitus (Type I diabetes, IDDM), and the remaining 90% have non-insulin-dependent diabetes mellitus (Type II diabetes, NIDDM). The long-term consequences of diabetes include increased risk of heart disease, blindness, end-stage renal disease, and non-healing ulcers in the extremities. The economic impact of diabetes to society has been estimated by the American Diabetes Association at approximately $45.2 billion annually (Jonsson, B., The Economic Impact of Diabetes, Diabetes Care 21(Suppl 3): C7-C10, (1998)).
A major long-term clinical study, the Diabetes Control and Complications Trial, involving 1,441 patients with insulin-dependent diabetes mellitus (Type I diabetes) over a 10-year period from 1984-1993, demonstrated that by intensive therapy (frequent administration of either short- or long-acting insulin), these long-term consequences (retinopathy, nephropathy, and neuropathy) could be reduced (“The Effect of Intensive Treatment of Diabetes on the Development and Progression of Long-Term Complications in Insulin-Dependent Diabetes Mellitus,” The Diabetes Control and Complications Trial Research Group, New Eng. J. Med., 329: 977-86 (1993)). Unfortunately, a major difficulty encountered during the trial was that intensive treatment also resulted in a higher incidence of low blood glucose levels (hypoglycemia), which was severe enough to result in coma or death, as compared to patients under conventional medical management.
Currently, diabetics must monitor their condition by repeatedly pricking their fingers in order to obtain blood samples for evaluation. The major drawback to self-monitoring of glucose is that it is discontinuous and therefore the number of glucose measurements performed is dependent on the motivation of the patient.
Existing analytical techniques and devices for in vitro glucose measurements have a high level of accuracy (the error can be <1%). Many of these routine methods are accepted as standards of comparison with new devices. Management of diabetes currently relies on these methods to control the disease and minimize complications.
There are two main disadvantages to these existing options. First, sampling even a minimal amount of blood multiple times per day is associated with risks of infection, nerve and tissue damage, and discomfort to the patients. Second, in the case of dynamic changes in glucose concentration, very frequent or even continuous measurements of blood glucose levels are required (Wilkins, E., et al., “Glucose Monitoring: State of the Art and Future Possibilities”, Med. Eng. Phys. 18(4):273-88, (1996).
There are two main approaches to the development of a continuous blood glucose monitor. The first category is non-invasive sensors, which obtain information from physico-chemical characteristics of glucose (spectral, optical, thermal, electromagnetic, or other). The second category is invasive sensors. In this group, there is intimate mechanical contact of the sensor with biological tissues or fluids, since the device is placed within the body. (Wilkins, 1996).
Non-invasive sensor technology has focused on the absorption of the near-infrared (NIR) spectra by the analyte, in this case, glucose (See U.S. Pat. No. 5,945,676 to Khalil, et al., and U.S. Pat. No. 5,433,197 to Stark). Absorptions which occur in the NIR region are most often associated with overtone and combination bands of the fundamental vibrations of —OH, —NH, and —CH functional groups. As a result, most biochemical species will exhibit some absorption in the region of interest. Glucose measurements are usually performed in the spectra region from 4250 to 660 cm−1. These highly overlapping, weakly absorbing bands were initially thought to be too complex for interpretation and too weak for practical application. Improvements in instrumentation and advances in multivariate chemometric data analysis techniques may allow meaningful results to be obtained from these complex spectra.
However, to date these devices are not particularly accurate even in the normal physiological range. A subject-dependent concentration bias has been reported. The temperature sensitivity of water absorption bands in the glucose-measuring region can be a significant source of error in clinical assays. In addition, the devices can also be affected by individual variations between patients at the measurement site. Skin location, temperature and tissue structure may affect the results, and decrease the accuracy of the reading.
Other investigators have looked into measurement of glucose from body fluids other than blood, such as sweat, saliva, urine, or tears. However, factors relating to diet and exercise can affect glucose levels in these fluids. In general, there is no strong correlation established between glucose concentration in the blood and in excreted fluids. The lag time between blood and excreted fluid glucose concentrations can be large enough to render such measurements inaccurate.
The continuous in vivo monitoring of glucose in diabetic subjects should greatly improve the treatment and management of diabetes by reducing the onus on the patient to perform frequent glucose measurements. Implanted glucose sensors could be used to provide information on continuously changing glucose levels in the patient, enabling swift and appropriate action to be taken. In addition, daily glucose concentration measurements could be evaluated by a physician. An implantable sensor could also provide an alarm for hypoglycemia, for example, overnight, which is a particular need for diabetics. Failure to respond can result in loss of consciousness and in extreme cases convulsive seizures. Similarly, a hyperglycemic alarm would provide an early warning of elevated blood glucose levels, thus allowing the patient to check blood or urine for ketone bodies, and to avert further metabolic complications. (Jaffari, S. A. et al., “Recent Advances In Amperometric Glucose Biosensors For In Vivo Monitoring”, Physiol. Meas. 16:1-15 (1995)).
Invasive glucose sensors may be categorized based on the physical principle of the transducer being incorporated. Current transducer technology includes electrochemical, piezoelectric, thermoelectric, acoustic, and optical transducers.
In piezoelectric, thermoelectric, and acoustic (surface acoustic wave, SAW) sensors used for glucose measurement, an enzyme-catalyzed reaction is used to create a measurable change in a physical parameter detected by the transducer. The development of these sensors is at an early laboratory stage (Hall, E.; Biosensors, Oxford University Press. Oxford, 1990). Optical sensors are based on changes in some optical parameter due to enzyme reactions or antibody-antigen reactions at the transducer interface. Based on the nature of the monitoring process, they are densitometric, refractometric, or colorimetric devices. At present, none of them meets the selectivity requirements to sense and accurately measure glucose in real physiological fluids.
There is a significant body of literature regarding the development of electrochemical glucose sensors. These generally incorporate an enzyme, which selectively reacts with glucose. Examples of enzymes, which selectively react with glucose, are glucose oxidase (GOD), hexokinase, glucose-6-phosphate dehydrogenase (G-6-PD), or glucose dehydrogenase. Hexokinase is an enzyme that catalyzes the phosphorylation of glucose by ATP to form glucose-6-phosphate and ADP. 
Monitoring the reaction requires a second enzyme, glucose-6-phosphate dehydrogenase, in the following reaction: 
The formation of NADPH may be measured by absorbance at 340 nm or by fluorescence at 456 nm (Jaffari, 1995).
Glucose dehydrogenase is another enzyme, which may be used for monitoring glucose in the following reaction: The NADH generated is proportional to the glucose concentration.
Glucose oxidase is the most commonly used enzyme reported in the literature. Its reaction is relatively simple, inexpensive, and may be monitored using a variety of techniques.
These advantages have led to the extensive use of this enzyme in clinical analysis as well as its incorporation in the majority of prototype biosensor configurations. The reaction of glucose with this enzyme is a two-stage reaction:                1) -D-glucose+GOD(FAD)→glucono-δ-lactone+GOD(FADH2)        2) GOD(FADH2)+O2→GOD(FAD)+H2O2         3) glucono-δ-lactone+H2O→gluconic acid        
The overall reaction is usually expressed as:                4) β-D-glucose+O2+H2O→gluconic acid+H2O2 The reaction can therefore be monitored by the consumption of oxygen, the production of hydrogen peroxide, or the change in acidity due to the increase of gluconic acid.        
Despite the foregoing and other efforts in the art, a suitable continuous in dwelling glucose sensor has not yet been developed.
A critical factor in the design of an implanted sensor is the anatomical site in which it is implanted. A few investigators have developed monitoring systems, which can be placed within the vascular system. Armour et al. (“Application of Chronic Intravascular Blood Glucose Sensor in Dogs”, Diabetes 39:1519-26 (1990)) implanted a sensor into the superior vena cava of six dogs for a period of up to 15 weeks with relative success. However, due to the risks of thrombosis and embolization, the majority of investigators have focused on subcutaneous implantation.
A major drawback to subcutaneous implantation is the body's defense against foreign objects: the “foreign-body response”. In this host response, if an object cannot be removed by the inflammatory response, foreign-body giant cells will form a “wall” around the object, which is subsequently followed by the formation of a fibrous capsule. If the object is a blood glucose sensor, it will no longer be in intimate contact with body fluids, and the signal will drift and stability will be lost. There are numerous reports of sensor stability being lost in about a week (Wilson, G. S., et al., “Progress Towards The Development Of An Implantable Sensor For Glucose”, Clin. Chem. 1992 38:1613-7, and Kemer, et al., “A Potentially Implantable Enzyme Electrode For Amperometric Measurement Of Glucose”, Horm. Metab. Res. Suppl. Ser. 20: 8-13 (1988)). Updike et al. (Updike, Stuart J., et al., “Enzymatic Glucose Sensors: Improved Long-Term Performance In Vitro And In Vivo”, ASAIO J., 40: 157-163 (1994)) reported on the subcutaneous implantation of a sensor which was stable for up to 12 weeks, however, this evaluation was only performed in three animals.
Recent clinical studies have also demonstrated that implantable insulin pumps are feasible for implantation for over one year (Jaremko, J. et al., “Advances Towards the Implantable Artificial Pancreas for Treatment of Diabetes,” Diabetes Care, 21(3): 444-450 (1998)). The research was inspired by the goal of the development of the artificial pancreas, and promising initial clinical trials using implantable insulin pumps. At this point in time, development of implantable insulin pumps is at a very advanced stage, with units being implanted for over 2 years in canines (Scavani et al., “Long-Term Implantation Of A New Programmable Implantable Insulin Pump,” Artif. Organs, 16: 518-22 (1992)) and in 25 patients for up to 3 years (Waxman, et al., “Implantable Programmable Insulin Pumps For The Treatment Of Diabetes”, Arch. Surg., 127: 1032-37 (1992)).
A number of wearable insulin pumps are described by Irsigler et al. (“Controlled Drug Delivery In The Treatment Of Diabetes Mellitus,” Crit. Rev. Ther. Drug Carrier Syst., 1(3): 189-280 (1985)). Thus, it should be relatively straightforward to couple a long-term implantable glucose sensor as described in this disclosure, to an insulin pump to optimize glycemic control for the patient.
Notwithstanding the extensive efforts in the prior art, however, there remains a need for an implantable blood glucose sensor for implantation in a blood vessel, which can provide useful blood glucose readings for an extended period of time, without material interference from thrombus formation, embolization, or other foreign body response. Preferably, the sensor is capable of continuous or near continuous monitoring, and driving an implantable insulin pump and/or making blood glucose data available to the patient or medical personnel.